As a method for fractionation of a fatty material into fractions having different melting points, solvent fractionation using a solvent such as acetone, hexane or the like has been known for long. Although fractionation can be effected precisely, this method is dangerous because a flammable solvent should be used and further this method is not preferred in view of industrial hygiene because the solvent uppers are detrimental to health. In addition, this method requires high production cost because of an increase in cost of a solvent consumed, heat energy cost for solvent recovery and installation cost of a solvent recovery apparatus.
As another fractionation method, emulsion fractionation using a surfactant is also known. However, this method has the a defect of low fractionation efficiency (yield and quality). In addition, post-treatment for separating a fat or oil from an aqueous solution containing a surfactant is complicated, and a big apparatus and high cost are required for treating waste water containing a surfactant.
As still another fractionation method, winterization has been known for long time. In this method, a solvent or surfactant is not used, and a fat or oil molten with heating is allowed to stand in a cooling tank at about 0.degree. C. to separate a lower solid layer from an upper liquid layer. This method requires many cooling tank facilities and takes several days until completion of fractionation. Therefore, productivity is low and fractionation efficiency is inferior to those of the above two methods.
Recently, in order to eliminate defects of these known fractionation methods, there has been proposed a fractionation method which comprises adding a hardened oil having a melting point of 55.degree. to 75.degree. C. as seeding crystals to a fat or oil molten with heating to accelerate crystallization, aging the mixture and filtering the mixture (Japanese Patent Kokoku No. 56-15759). This method is applicable when the amount of crystals after aging is relatively small (i.e., when a yield of a desired solid fraction is low). However, when an amount of crystals becomes larger, pumping of crystals becomes difficult and therefore handling thereof takes much time.
Further, as a method for fractionation of a fraction which is compatible with cacao butter from a vegetable fat such as mango kernel oil or sal fat, there has been proposed a dry fractionation method of a fat which comprises adding .beta. or .beta.' crystals obtained from a kernel fat or cacao fat as seed crystals to a vegetable fat molten with heating, aging the mixture to form the desired stable crystals, gradually cooling to a fractionation temperature to grow crystals and filtering the mixture under pressure (Japanese Patent Kokai No. 60-101197). However, in this dry fractionation method, the resulting crystal aggregate has a paste-like consistency. Therefore, in order to effect fractionation, the paste-like crystal aggregate should be transferred from a tank or tray into bags suitable for filtration by pressing and the bags are piled up in a press to apply a pressure from the top to squeeze out a liquid fraction. This takes much time and requires a great deal of labor.